John Huth Harvard University

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1 John Huth Harvard UniversityWeather and WavesJohn HuthHarvard University

2 Weather Basics Hot air rises (less dense), cold air sinks (more dense)Atmosphere becomes colder the higher up you go (called adiabatic cooling)It gets colder as you go away from the equatorThe Coriolis effect causes air moving away from the equator to the pole to deflect to the eastThe Coriolis effect causes air moving from the pole toward the equator to deflect to the west

3 Driving Forces Behind WindPressure GradientAir flows from high to low pressure (“downhill”)CoriolisCaused by the rotation of the earth, wind deflects to the right in the northern hemisphereCentripitalPresent when winds are in rotationFrictionAir moving along the Earth’s surface is slowed by friction

19 Using wind Winds can be deceivingSurface winds can blow in different directions from winds aloft – you must follow the motion of high clouds to get prevailing windsWinds will shift as fronts pass through – knowledge of this is important (for many reasons).Safety – high winds from thunderstorms can be dangerous when at sea.

21 Warm and Cold Air massesWarm air massesHumid, low pressure, warm - move up from equatorial regionsCold air massesDry, high pressure, cold – move down from polar regionsTransitions between air masses are called “fronts”

22 Weather signsCloud formations and wind directions are the most reliable and predictive (often better than NOAA radio).Best predictor: tomorrow will be like today (true 80% of the time). You can improve on this by being observant.Some signs: “red sky at night” are next to useless – unless you know the cloud formations causing them.

50 Multicell line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. An approaching multicell line often appears as a dark bank of clouds covering the western horizon. The great number of closely-spaced updraft/downdraft couplets qualifies this complex as multicellular, although storm structure is quite different from that of the multicell cluster storm.

51 Estimating distances to stormsBase of clouds in thunderstorm is typically 5000 ft.Use range techniques to find distanceDifference between lightning and thunder arrival times (light is faster than sound)5 seconds per mile of distancePrevailing winds –Is the storm track moving toward you, or will it pass by?

52 Thunderstorm/squall issuesGeneral direction is indicated by high cirrus clouds at top of anvil headNOT surface winds (often blow toward the storm)If a storm misses you (passes to the side), be alert for more storms moving in the same direction.Wind is biggest issueLightning is less of a hazard, but shouldn’t be ignored.

65 Deep Water Waves (Figure 7-4a)Waves do not interact with the seafloorOrbits of the water molecules are circular.

66 Shallow Water Waves (Figure 7-4b)Waves interact with the seafloor are known as Orbits of the water molecules become elliptical.

67 Characteristics of water wavesVelocity depends on wavelength *or* water depthUnlike sound or light – velocity is independent of wavelength for theseWaves become unstable when height is 1/7th of wavelength – whitecaps (120 degree interior angle)Longer wavelength waves hold more energyDepth for “shallow” versus “deep” is about 2 times wavelength

72 Wind Generation of WavesThe type of wave generated by wind is determined by:Wind velocityWind durationFetch (distance over which wind blows)Simply put, wave size increases as the strength and duration of the wind, and distance over which it blows increases.